1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving data in a multi-user multi-antenna communication system. In particular, the present invention relates to a data transmitting apparatus and method for computing a transmit (TX) filter of a transmitter in an improved scheme and transmitting the computed TX filter to a receiver in a one-way channel sounding scheme; and a data receiving apparatus and method for receiving the TX filter over a channel and using the product of the received TX filter and a channel matrix as a receive (RX) filter.
2. Description of the Related Art
Research has been conducted on Multiple Input Multiple Output (MIMO) channels over ten years. In addition, researches are being conducted on a multi-user multi-antenna communication system. The use of multi-antenna in a transmitter increases spectral efficiency and also the use of a multi-antenna in a receiver further increases spectral efficiency. Research on the multi-user multi-antenna communication system is intended to apply the above fact to a multi-user communication system.
A MIMO communication system establishes a multi-link between a transmitter and a single user, thereby increasing spectral efficiency. In the MIMO communication system, only one user can access a given resource among channel resources (e.g., subcarriers, spreading codes, and cell sectors) at a time. That is, a MIMO link (i.e., independent data streams) exists between a transmitter and only one receiver at a given time. On the other hand, the multi-user multi-antenna communication system allows a plurality of users (terminals) to access the same resource simultaneously, and independent data streams occur between a transmitter and a plurality of user receivers. The multiplexing scheme used for this is called “multi-user Spatial Multiplexing (SM)”.
FIG. 1 is a schematic block diagram of a multi-user multi-antenna communication system where communications are performed between a transmitter having a plurality of TX antennas and a plurality of receivers each having a plurality of RX antennas.
Referring to FIG. 1, a base station (BS) transmitter 110 having N number of TX antennas (simply referred to as “N TX antennas”) communicates with K user receivers 120, 130 and 140 each having a plurality of RX antennas. The transmitter 110 transmits independent data streams to the receivers 120, 130 and 140 by using a multi-user SM scheme.
A MIMO broadcast channel, an example of a multi-user multi-antenna communication channel, is a downlink (DL) channel of a cellular network where a base station (transmitter) uses multiple TX antenna.
An example of a theoretical scheme for a MIMO broadcast channel in a cellular system is disclosed in M. Costa, “Writing On Dirty Paper”, IEEE Transactions on Information Theory, Vol. 29, No. 3; pp. 439-441, May 1983. This theoretical scheme is, however, unrealistic because it is based on the assumption that a transmitter and every user receiver accurately know the channels of all user receivers. In addition, the above theoretical scheme is highly complex because it uses non-linear precoding techniques.
FIG. 2 is a block diagram of a multi-user multi-antenna communication system where a transmitter uses a plurality of TX antennas and a TX filter and a plurality of receivers each using a plurality of RX antennas and an RX filter. For actual communication, a transmitter uses a TX filter and each receiver uses an RX filter.
Referring to FIG. 2, the transmitter includes SM TX filters 210, 220 and 230 that incorporate TX filters Ml, Mk, . . . , Mn, respectively. Likewise, the receivers respectively include SM RX filters 240, 250 and 260 that incorporate RX filters Wl, Wk, . . . , Wn, respectively.
A multi-user SM scheme, disclosed in Lai-U Choi and Ross D. Murch, “A Transmit Preprocessing Technique for Multiuser MIMO Systems Using a Decomposition Approach”, IEEE Transactions on Wireless Communications, Vol. 2, No. 4, pp. 773-786, July 2003, is based on the assumption that some entity (terminal) knows channel matrixes of all users (terminals) and can compute TX/RX filters for optimization of communication performance.
The disclosed multi-user SM scheme is, however, silent on which entity can compute the TX/RX filters, and on how the computed TX/RX filter information (knowledge) can be transmitted from a transmitter to receivers. If information (knowledge) about all the respective channel matrixes between the transmitter and the receivers is available to the receiver, it is called “global channel information (knowledge)”. This is, however, also impossible in an actual system.
FIG. 3 is a graph illustrating spectral efficiencies depending on the type of algorithm used in a multi-user multi-antenna communication system. In FIG. 3, local Channel State Information (CSI) refers to a realistic case where each receiver knows only its own channel matrix. Global CSI refers to an unrealistic case where each receiver knows channel matrixes of all receivers. Partial CSI refers to a case where a transmitter uses some measure of channel. quality, which indicates CSI such as signal-to-noise ratio (SNR). Complete CSI refers to a case where a transmitter uses the complex entry of CSI in itself.
As can be seen from FIG. 3, the frequency efficiency in the case of transmission to a single user (single-user closed loop, local CSI, complete CSI), as illustrated mark +, is lower by 4.6 bits/sec/Hz than that in the case of sum-capacity (nonlinear precoding scheme; global CSI, complete CSI), as illustrated mark ∇, which is the theoretical maximum data rate of a multi-user communication having four TX antennas, four users, and four RX antennas. Although not illustrated in FIG. 3, the frequency efficiency decreases when multiplex transmission is performed on user receivers but a transmitter and a receiver use partial CSI and local CSI, respectively.
The spatial efficiency in the case of a coordinated beamforming algorithm, as illustrated mark o, is lower by 0.7 bits/sec/Hz than that in the case of the sum-capacity. The use of the coordinated beamforming algorithm makes it possible to design an effective method of transmitting information about RX filters to user receivers by transmitting only one layer to each user receiver, as disclosed in B. Farhang-Boroujeny, Q. Spencer and L. Swindlehurst, “Layering Techniques for Space-Time Communication in Multi-User Networks,” in Proceedings of IEEE Vehicular Technology Conference (VTC'03 Fall), Orlando, Fla., Oct., 6-9, 2003, Vol. 2, pp. 1339-1343.
The coordinated beamforming algorithm is now described in detail.
First, an algorithm for computing TX/RX filters follows:
Computation Phase
Ai represents the ith column of any matrix A. When the matrix A is Singular Value Decomposition (SVD)-processed, A=UDV*. Here, U and V are unitary matrixes and D is a singular value of the matrix A with diagonal elements arranged in descending order. The principal left singular vector of the matrix A is denoted as Ul that is the first column of the matrix U.
The following computation is performed on the assumption that a base station has a complete CSI.
Initializationfor k = 1:KHk = UDV   SVDWk = U1endRepeat times:Effective channel vector computationfor k = 1:K      H          eff      ,      k        =            W      k      *        ⁢          H      k      endUpdate of TX/RX filtersfor k = 1:K      H          stacked      ,      k        =            [                                    H                          eff              ,              1                        T                    ⁢                                          .                                          .                                          .                                          ⁢                      H                          eff              ,                              k                -                1                                      T                          ⁢                                  ⁢                              H                          eff              ,                              k                +                1                                      T                    ⁢                                          .                                          .                                          .                                          ⁢                      H                          eff              ,              K                        T                              ]        T  Hstacked,k = UkDkVk   SVDMk = Vk,N       W    k    =                    H        k            ⁢              V                  k          ,          N                                                    H          k                ⁢                  V                      k            ,            N                                     endEnd of iterations
where SVD represents singular value decomposition, D is a diagonal matrix, and U and V are unitary matrixes. Heff,k represents an effective channel matrix of a user receiver k that the receiver actually experiences, and Hstacked,k represents effective channels of receivers of all users other than the user receiver k. Mk is a TX filter matrix for the user receiver k, Wk is an RX filter matrix for the user receiver k, and Vk,n is a singular vector in the num space of Hstacked,k. T represents a transpose, and * represents a complex conjugate transpose.
As the algorithm is iterated to convergence, a transmitter performs zero-forcing beamforming (ZFBF) on each user receiver based on an effective channel matrix containing an RX filter.
Although not indicated in the above algorithm, SVD is again used to calculate Wk from Mk. As a result, the above algorithm performs SVD on each user receiver twice during the computation of the TX/RX filters and performs SVD once during the initialization, which causes a complexity problem. Moreover, the base station knows the optimal TX/RX filters for each of N user receivers but the user receivers do not. However, the above algorithm is silent on a technique for informing the receivers of the optimal RX filters. What is therefore required are: (a) a scheme for informing the receivers of the optimal RX filters;
(b) a data transmitting apparatus and method for a multi-antenna communication system that can provide a simpler scheme for computing a TX filter and an improved scheme for enabling a receiver to easily compute an RX filter; and (c) a data receiving apparatus and method for a multi-antenna communication system that can efficiently receive a TX filter computed by a transmitter. Finally, what is required is a data transmitting/receiving apparatus and method for a multi-user multi-antenna communication system that can reduce the system complexity, increase the spectral efficiency and be implemented even when a receiver does not know the channels of other receivers.